chore: track the first time html is served in telemetry (#16334)

Addresses https://github.com/coder/nexus/issues/175.

## Changes

- Adds the `telemetry_items` database table. It's a key value store for
telemetry events that don't fit any other database tables.
- Adds a telemetry report when HTML is served for the first time in
`site.go`.

coderd/database/queries/telemetryitems.sql

@@ -0,0 +1,15 @@
+-- name: InsertTelemetryItemIfNotExists :exec
+INSERT INTO telemetry_items (key, value)
+VALUES ($1, $2)
+ON CONFLICT (key) DO NOTHING;
+
+-- name: GetTelemetryItem :one
+SELECT * FROM telemetry_items WHERE key = $1;
+
+-- name: UpsertTelemetryItem :exec
+INSERT INTO telemetry_items (key, value)
+VALUES ($1, $2)
+ON CONFLICT (key) DO UPDATE SET value = $2, updated_at = NOW() WHERE telemetry_items.key = $1;
+
+-- name: GetTelemetryItems :many
+SELECT * FROM telemetry_items;